ISO BYZANTINE CHURCHES CHAP. north is covered with a cross-groined vault of lath and plaster, probably on the model of an original vault constructed of brick. A door in the eastern wall leads to the north aisle of the church. The southern bay is separated from its companion by a broad arch. It is an oblong chamber reduced to a figure approaching a square by throwing broad arches across its ends and setting back the wall arches from the cornice. This arrangement allows the bay to be covered by a low drumless dome. Two openings, separated by a pier, lead respectively to the nave and the southern aisle of the • church. The interior of the church has undergone serious alterations since it has become a mosque, but enough of the original building has survived to show that the plan was that of an c ambulatory church.' Each side of the ambulatory is divided into three bays, covered with cross-groined vaults whose springings to the central area correspond exactly to the columns of such an arcade as . that which occupies the west dome bay ot S. Andrew (p. 114). We may therefore safely assume that triple arcades originally separated the ambulatory from the central area and filled in the lower part of the dome arches. The tympana of these arches above were pierced to north, south, and west by three windows now built up but whose outlines are still visible beneath the whitewash which has been daubed over them. The angles of the ambulatory are covered by cross vaults. The pointed arches at present opening from the ambulatory to the central area were formed to make the church more suitable for Moslem worship, as were those of the north church. In fact we have here a repetition of the treatment of the Pammakaristos (p. 151), when converted into a mosque. The use of cross-groined vaults in the ambulatory is a feature which distinguishes this church from the other ambulatory churches of Constantinople and connects it more closely with the domed-cross church. The vaults in the northern portion of the ambulatory have been partially defaced in the course of Turkish repairs. The central apse is lighted by a large triple window.